Carbon black can be successfully produced by pyrolytically decomposing a hydrocarbon feedstock, such as natural gas or petroleum oil, in a furnace. The heat for this thermal decomposition is usually generated by hot combustion gases, such as the combustion products of natural gas and air. One furnace that has proven to be very efficient in producing carbon black from a hydrocarbon oil feedstock is the vortex flow carbon black reactor. This reactor has a cylindrical precombustion zone in open communication with a cylindrical reaction zone. The axis of rotation of the cylindrical precombustion zone and the axis of rotation of the cylindrical reaction zone are substantially collinear. The diameter of the precombustion zone is greater than that of the reaction zone. The reactor also includes a means for axially introducing a hydrocarbon feedstock into the reactor and a means for tangentially introducing combustion gases into the precombustion zone. Thermal decomposition of the feedstock occurs by contact with the combustion gases.